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                    <i class="far fa-calendar-minus fa-fw"></i>发布日期:&nbsp;&nbsp;
                    2020-03-12
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                <h2 id="第二章-日期时间类"><a href="#第二章-日期时间类" class="headerlink" title="第二章 日期时间类"></a>第二章 日期时间类</h2><p>2.1 Date类<br>    表示日期时间的类，精确到毫秒，特定的瞬间<br>    当前日期：2088-01-01<br>    时间原点：1970-01-01 00:00:00(英国格林威治)</p>
<span id="more"></span>

<p>public class Demo01Date{<br>    public static void main(String[] args){<br>        System.out.println(System.currentTimeMiLLis());&#x2F;&#x2F;获取时间原点到当前时间经历了多少毫秒</p>
<pre><code>&#125;
</code></pre>
<p>}</p>
<p>public class Demo02Date {<br>    public static void main(String[] args) {<br>&#x2F;&#x2F;		demo01();<br>        demo02();<br>        demo03();<br>    }</p>
<pre><code>//成员方法 Long getTime() 把日期转换为毫秒
private static void demo03() &#123;
    Date date = new Date();
    long time = date.getTime();
    System.out.println(time);//1583119265044
&#125;

//带参数的构造方法
//Date(Long date) 传递毫秒值，把毫秒值转换为Date日期
private static void demo02() &#123;
    Date date = new Date(1583119265044L);
    System.out.println(date);
&#125;

// Date类空参数构造方法
private static void demo01() &#123;
    Date date = new Date();
    System.out.println(date);

&#125;
</code></pre>
<p>}</p>
<p>2.2 DateFormat类<br>    java.text<br>    日期时间格式化子类的抽象类</p>
<p>public class Demo01DateFormat {<br>    &#x2F;*<br>    成员方法：<br>    String format(Date date) 按照指定模式，把Date日期格式化为符合模式的字符串<br>    Date parse(String source) 把符合模式的字符串解析为Date日期 throws 抛出异常<br>    构造方法：<br>    SimpleDateFormat(String pattern) 用给定模式构造<br>    y 年 M 月 d 日 H 时 m 分 s 秒<br>    写对应的模式，会把模式替换为对应的日期和时间<br>    ‘yyyy-MM-dd HH:mm:ss’<br>    注意：<br>    模式中的字母不能改，连接模式的符号可以改<br>    ‘yyyy年MM月dd日 HH时mm分ss秒’</p>
<pre><code> */

public static void main(String[] args) throws ParseException &#123;
    demo01();
    demo02();
&#125;

private static void demo01() &#123;
    SimpleDateFormat sdf = new SimpleDateFormat(&quot;yyyy年MM月dd日 HH时mm分ss秒&quot;);
    Date date = new Date();
    String d = sdf.format(date);
    System.out.println(date);
    System.out.println(d);

&#125;

private static void demo02() throws ParseException &#123;
    SimpleDateFormat sdf = new SimpleDateFormat(&quot;yyyy年MM月dd日 HH时mm分ss秒&quot;);
    Date date = sdf.parse(&quot;2020年03月02日 13时05分54秒&quot;);
    System.out.println(date);

&#125;
</code></pre>
<p>}</p>
<p>2.4 Calendar类<br>    是一个抽象类，里面提供了很多操作日历字段的方法<br>    无法直接创建对象使用，里面有一个静态方法叫个体Instance(),该方法返回Calendar子类对象</p>
<pre><code>/*
</code></pre>
<p>常用成员方法：<br>public int get(int field): 返回给定日历字段的值<br>public void set(int field,int value): 将指定日历字段设置为给定值<br>public abstract void add(int field,int amount): 根据日历的规则，为给定日历字段添加或减去指定的时间量<br>public Date getTime(): 返回一个表示此Calendar时间值的Date对象</p>
<p> *&#x2F;<br>public class CalendarClass {<br>    public static void main(String[] args) {</p>
<pre><code>    demo01();
&#125;

private static void demo01() &#123;
    Calendar c = Calendar.getInstance();
    System.out.println(c);

    int year = c.get(Calendar.YEAR);
    System.out.println(year);
    int month = c.get(Calendar.MONTH) + 1;//计算机中从0开始
    System.out.println(month);
    int date = c.get(Calendar.DATE);
    System.out.println(date);

    //set 方法
    c.set(Calendar.YEAR, 200);
    c.set(Calendar.MONTH, 3);
    int year2 = c.get(Calendar.YEAR);
    System.out.println(&quot;after setting&quot; + year2);
    int month2 = c.get(Calendar.MONTH) + 1;//计算机中从0开始
    System.out.println(&quot;after setting&quot; + month2);
    //同时设置年月日

    c.set(1997, 3, 10);
    int year3 = c.get(Calendar.YEAR);
    System.out.println(&quot;after setting&quot; + year3);
    int month3 = c.get(Calendar.MONTH) + 1;//计算机中从0开始
    System.out.println(&quot;after setting&quot; + month3);

    //add方法
    c.add(Calendar.YEAR, 8);
    c.add(Calendar.MONTH, -2);
    int year4 = c.get(Calendar.YEAR);
    System.out.println(&quot;after setting&quot; + year4);
    int month4 = c.get(Calendar.MONTH) + 1;//计算机中从0开始
    System.out.println(&quot;after setting&quot; + month4);

    //getTime方法
    Date d = c.getTime();
    System.out.println(d);

&#125;
</code></pre>
<p>}</p>
<h2 id="第三章-System类"><a href="#第三章-System类" class="headerlink" title="第三章 System类"></a>第三章 System类</h2><p>提供大量静态方法，可以获取与系统相关的信息或系统级操作<br>常用方法：<br>public static long currentTimeNillis(): 返回以毫秒为单位的当前时间<br>public static void arraycopy(Object src,int srcPos,Object dest,int destPos,int,length): 将数组中指定数据拷贝到另一个数组中</p>
<p>public class Demo01System {<br>    public static void main(String[] args) {<br>        demo01();<br>        demo02();<br>    }</p>
<pre><code>private static void demo01() &#123;
    //public static long currentTimeNillis;//返回以毫秒为单位的当前时间,用来看程序的效率
    //程序开始前，获取一次
    long s = System.currentTimeMillis();
    //执行for循环
    for (int i = 1; i &lt;= 9999; i++) &#123;
        System.out.println(i);
    &#125;

    //程序执行后，获取一次毫秒值
    long e = System.currentTimeMillis();
    System.out.println(&quot;total&quot;+(e-s)+&quot;ms&quot;);
&#125;


private static void demo02() &#123;
    /*public static void arraycopy(Object src,int srcPos,Object dest,int destPos,int,length)
    src 源数组
    srcPos 源数组中的起始位置(起始索引)
    dest 目标数组
    destPos 目标数组中的起始位置
    length 要复制的数组元素的数量
     */

    // 将src前三个数字复制到dest前三个位置上
    int [] src = new int[]&#123;1,2,3,4,5&#125;;
    int [] dest = &#123;6,7,8,9,0&#125;;
    System.out.println(&quot;before copying&quot;+ Arrays.toString(dest));
    System.arraycopy(src,0,dest,0,3);
    System.out.println(&quot;after copying&quot;+ Arrays.toString(dest));
    
&#125;
</code></pre>
<p>}</p>
<h2 id="第四章-StringBuilder类"><a href="#第四章-StringBuilder类" class="headerlink" title="第四章 StringBuilder类"></a>第四章 StringBuilder类</h2><p>String类是常量，创建之后不可改变。进行字符串相加，内存中就会有多个字符串，占用空间多，效率低下<br>StingBuilder类 字符串缓冲区，可以提高字符串的操作效率底层是一个数组，但没有被final修饰，可以改变长度<br>StringBuilder在内存中始终是一个数组，占用空间少，效率高。初始长度16，如果超出容量，会自动扩容</p>
<p>public class StringBuilderClass {<br>    &#x2F;*<br>    构造方法：<br>        StringBuilder() 无参构造<br>        StringBuilder(String str) 带参构造，初始化为指定字符串内容，可用于String-&gt;StringBuilder<br>    常用成员方法：<br>     	public StringBuilder append(): 添加任意类型数据的字符串形式，并返回当前对象自身<br>     	public String toString(): 将当前StringBuilder对象转换为String，可用于StringBuilder-&gt;String</p>
<pre><code> */

public static void main(String[] args) &#123;
    //无参构造
    StringBuilder bu = new StringBuilder();
    System.out.println(&quot;bu:&quot;+bu);

    //带参构造
    StringBuilder bu2 = new StringBuilder(&quot;abc&quot;);
    System.out.println(&quot;bu2:&quot;+bu2);

    StringBuilder bu1 = bu.append(&quot;abc&quot;);//把bu地址赋值给了bu1
    System.out.println(bu);//&quot;abc&quot;
    System.out.println(bu1);//&quot;abc&quot;
    System.out.println(bu==bu1);//比较的是地址，返回true
    //实际上使用append方法无需接受返回值
    bu.append(1);
    System.out.println(bu);
    //链式编程，返回的是对象，可以直接再调用方法
    bu.append(&quot;China&quot;).append(&quot;No.&quot;).append(&quot;1&quot;);
    System.out.println(bu);


    //toSting方法
    String str = &quot;hello&quot;;
    System.out.println(&quot;str:&quot;+str);
    StringBuilder builder = new StringBuilder(str);
    builder.append(&quot;world&quot;);
    System.out.println(&quot;builder:&quot;+builder);
    String s = builder.toString();
    System.out.println(&quot;s:&quot;+s);

&#125;
</code></pre>
<p>}</p>
<p>&#x2F;&#x2F; .var 自动生成变量接收当前对象  new StringBuilder(“abc”).var &#x3D;&gt;StringBuilder bu2 &#x3D; new StringBuilder(“abc”);</p>
<h2 id="第五章-包装类"><a href="#第五章-包装类" class="headerlink" title="第五章 包装类"></a>第五章 包装类</h2><p>Java 提供了两个类型系统，基本类型和引用类型，使用基本类型在于效率，然而很多情况，会创建对象使用<br>因为对象可以做更多的功能，如果想要基本类型像对象一样操作，就可以使用基本类型对应的包装类<br>基本类型：byte, short, int, long, float, double, char, boolean<br>包装类：Byte, Short, Integer, Long, Float, Double, Character, Boolean</p>
<p>装箱拆箱<br>装箱：把基本类型的数据，包装到包装类中<br>    构造方法：<br>        Integer(int value) 构造一个新分配的Integer对象，它表示指定的int值<br>        Integer(String s) 构造一个新分配的Integer对象，它表示String参数所指示的int值<br>            传递的字符串，必须是基本类型的字符串，否则抛出异常，‘100’正确，‘a’异常<br>    静态方法：<br>        static Integer valueOf(int i) 返回一个表示指定的int值的Intger实例<br>        static Integer valueOf(String s) 返回保存指定的String 的值的Integer对象</p>
<p>拆箱：在包装类中取出基本类型的数据，包装到包装类中<br>    成员方法：<br>    int intValue() 以int类型返回该Integer的值</p>
<p>public class Demo01Integer {<br>    public static void main(String[] args) {<br>        &#x2F;&#x2F;装箱<br>        Integer in1 &#x3D; new Integer(1);&#x2F;&#x2F;1<br>        System.out.println(in1);&#x2F;&#x2F;重写了toString方法<br>        Integer in2 &#x3D; new Integer(“1”);<br>        System.out.println(in2);&#x2F;&#x2F;1</p>
<pre><code>    //静态方法
    Integer in3 = Integer.valueOf(&quot;1&quot;);
    System.out.println(in3);

    //拆箱
    int i = in1.intValue();
    System.out.println(i);
    
    //自动装箱
    Integer in = 1;
    //自动拆箱
    in = in+2;//in是包装类，无法运算，会自动转换成int

&#125;
</code></pre>
<p>}</p>
<p>基本类型与字符串类型之间的互相转换<br>基本类型-&gt;字符串<br>    1. 基本类型的值 +”” 最简单的方法<br>    2. 包装类的静态方法toString(参数)，不是Obeject类的toString()重载<br>    3. String类的静态方法valueOf(参数)<br>        static String valueOf(int i) </p>
<p>字符串-&gt;基本类型<br>        使用包装类的静态方法parseXXX(“数值类型的字符串”)<br>        Integer类：static int parseInt(String s)<br>        Double类：static double parseDouble(String s)</p>

                
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